True dietary protein consists of amino acids joined together in various combinations. In the field of ruminant livestock nutrition, it is known that under some circumstances protecting dietary protein from extensive degradation in the rumen by microbial enzymes can lead to an increase in the outflow of amino acids from the rumen and/or a change in the balance of amino acids reaching the lower gut. In circumstances where the supply or balance of amino acids reaching the lower gut is metabolically limiting, supplementation of the diet with protein that is partially protected from degradation by rumen microbes can improve productivity in terms of improved efficiency of meat, milk and wool or hair production. Specifically, increased production of these products may be achieved with a given dietary protein content where part of this protein content is protected from ruminal degradation or, alternatively, equivalent levels of production of these products may be achieved with reduced true dietary protein content where part of this protein content is protected from ruminal degradation.
Various methods have been used for protecting proteins from ruminal degradation including the simple application of heat. In the following discussion, reference is made to the list of publications given at the end of this specification. Chemical agents such as formaldehyde (Reis and Tunks, 1969), alcohol (van de Aar et al,1982), bentonites (Britton et al, 1978), zinc (Britton and Klopfenstein, 1986), tannins (Driedger and Hatfield, 1972), and sodium hydroxide (Mir et al, 1984) have also been used successfully to treat protein as a means of reducing ruminal degradability. All these methods of treatment, including heating, are thought to act either by inhibiting proteolytic activity and/or by modifying protein structure in such a way that the number of protease specific bonds that can be cleaved by microbial enzymes is decreased.
A specific method of improving the efficiency of utilization of protein by ruminants utilizing a mixture of a reducing carbohydrate and proteinaceous material was first described in Cleale, R. M., T. J. Klopfenstein, R. A. Britton and L. D. Satterlee, 1986, "Induced non-enzymatic browning of soybean meal for enhancing efficiency of protein utilization by ruminants", J. Anim. Sci. 63 (Suppl. 1):139, (Abstr.). Subsequently, U.S. Pat. No. 4,957,748 and U.S. Pat. No. 5,064,665 disclosed methods of preparing feeds for ruminants that include proteins and reducing carbohydrates or the reaction products between these compounds. None of these methods suggest the application of pressure and shear forces to improve the degree of protein protection and hence increase the efficiency of utilization of the protein by ruminant animals. U.S. Pat. No. 4,957,748 and U.S. Pat. No. 5,064,665 also cite examples of soybean meal treated with Xylose (a reducing carbohydrate) such that the ruminal escape of the protein content was approximately 2.6 times that of commercial untreated soybean meal when measured in an in vivo trial. These documents also suggest improvements in protein use efficiency for proteins subjected to early
Mallard type reactions of at least 50% and in some circumstances 100%.
Methods utilizing reactions between proteins and reducing carbohydrates to protect protein from ruminal degradation basically involve the Maillard reaction, a well known chemical reaction also known as non-enzymatic browning. The first stage of this reaction involves condensation of a reducing sugar and an amino acid contained in a protein molecule to form a Schiff base. The next stage of the reaction involves rearrangement to form a more stable Amadori product. If further reaction takes place indigestible melanoidins may be formed. For the protein to be available to the animal in the lower digestive tract it is thought to be necessary to arrest the Maillard reaction in its early stages.
The rate and extent of formation of Maillard products is known to be dependent on the temperature, pH and water content of the material. The rate and extent also depends upon the particular reducing carbohydrate present and the amino acid composition of the protein. It is also probable that the tertiary structure of the protein may inhibit the extent of the reaction especially where reducing carbohydrates of higher molecular size are involved due to steric hindrance.
An extruder or expander barrel may be thought of as a reaction chamber where pressure, physical forces and heat are concurrently applied to material passing through the machine. These factors may accelerate and/or modify chemical reactions. The heat, pressure and shear forces involved in extrusion are also know to disrupt the tertiary structure of proteins by breaking secondary bonds which may reduce steric hindrance in Maillard type reactions. Extrusion may also cause cross linking within the protein structure that partially protects it from ruminal degradation and reduces the number of cleavable sites in the molecule, hence reducing the amount of chemical reagent needed to protect the remainder of these sites if further protection is required. Extrusion per se has also been shown to reduce ruminal protein degradability without markedly reducing amino acid absorption in the lower digestive tract (Stern et al, 1980).
Cros, P., R. Moncoulon, C. Bayourthe and M. Vernay, 1992,"Effect of Extrusion on Ruminal and Intestinal Disappearance of Amino Acids in White Lupin Seed", Canadian Journal of Animal Science 72:89-96, describes the effect of extruding white lupin seed at 120 or 150.degree. C. It was found that protein degradability in Nylon bags incubated in the rumens of Holstein cattle for sixteen hours was reduced to 73.6% for the higher extrusion temperature compared to 98.4% for the unprocessed lupin control. It was also found that the amino acid composition of the extruded protein that escaped ruminal digestion differed markedly both quantitatively and qualitatively from its initial composition. Comparisons with milk protein composition enabled the authors to postulate that the ruminally undegraded protein that subsequently disappeared in the intestine showed a higher protein quality than the original protein.
For a treatment to be beneficial in practice it is important to achieve maximum protection of dietary protein from ruminal degradation without markedly reducing lower gut absorption of its amino acid content, at minimum cost. It is also important to obtain a predictable result, and it would be beneficial if the biological value of the protein that escapes ruminal degradation were higher than the biological value of the original dietary protein.
The following descriptions will show that the present invention offers a more effective method of protecting protein than either simple extrusion, or use of Maillard type reactions without the concurrent application of pressure and shear forces.